A&A 473, 847–855 (2007) Astronomy DOI: 10.1051/0004-6361:20077960 & c ESO 2007 Astrophysics A comprehensive study of Cepheid variables in the Andromeda galaxy Period distribution, blending, and distance determination F. Vilardell1,C.Jordi1,3, and I. Ribas2,3 1 Departament d’Astronomia i Meteorologia, Universitat de Barcelona, c/ Martí i Franquès, 1, 08028 Barcelona, Spain e-mail: [francesc.vilardell;carme.jordi]@am.ub.es 2 Institut de Ciències de l’Espai-CSIC, Campus UAB, Facultat de Ciències, Torre C5-parell-2a, 08193 Bellaterra, Spain e-mail: [email protected] 3 Institut d’Estudis Espacials de Catalunya (IEEC), Edif. Nexus, c/ Gran Capità, 2-4, 08034 Barcelona, Spain Received 28 May 2007 / Accepted 18 July 2007 ABSTRACT Extragalactic Cepheids are the basic rungs of the cosmic distance scale. They are excellent standard candles, although their lumi- nosities and corresponding distance estimates can be affected by the particular properties of the host galaxy. Therefore, the accurate analysis of the Cepheid population in other galaxies, and notably in the Andromeda galaxy (M 31), is crucial to obtaining reliable distance determinations. We obtained accurate photometry (in B and V passbands) of 416 Cepheids in M 31 over a five year campaign within a survey aimed at the detection of eclipsing binaries. The resulting Cepheid sample is the most complete in M 31 and has almost the same period distribution as the David Dunlap Observatory sample in the Milky Way. The large number of epochs (∼250 per filter) has permitted the characterisation of the pulsation modes of 356 Cepheids, with 281 of them pulsating in the fundamental mode and 75 in the first overtone. The period-luminosity relationship of the fundamental mode Cepheids has been studied and a new approach has been used to estimate the effect of blending. We find that the blending contribution is as important as the metallicity correction when computing Cepheid distance determinations to M 31 (∼0.1 mag). Since large amplitude Cepheids are less affected by blending, we have used those with an amplitude AV > 0.8 mag to derive a distance to M 31 of (m − M)0 = 24.32 ± 0.12 mag. Key words. Stars: variables: Cepheids – stars: distances – galaxies: individual: M 31 – galaxies: distances and redshifts – methods: observational 1. Introduction (Udalski et al. 1999; Mochejska et al. 2001; Udalski et al. 2001; Pietrzynski´ et al. 2004; Macri et al. 2006), obtaining large sam- Cepheids are probably the most studied variable stars. Their ples of Cepheids with accurate photometry. The detailed study large amplitudes and intrinsic luminosities make them eas- of the observed Cepheids has emphasized the importance of an ily detectable in most photometric variability surveys. In ad- issue that was usually overlooked in most photometric stud- dition, their well-known period-luminosity (P-L) relationship ies: the effect of blending. It has been proposed (Mochejska (Leavitt & Pickering 1912) has made these variable stars one et al. 2000, hereafter MMSS00) that the magnitude of Cepheids of the main cornerstones in deriving extragalactic distances (see may be affected by the light of unresolved companion stars Freedman et al. 2001, for an historical review). The importance (i.e., blends). The effect of blending is somewhat different from of Cepheids for distance determination stands in contrast with crowding or confusion noise, since companion stars appear to be the relative lack of additional information on the specific char- in the same point-like source. Therefore, even when achieving acteristics of extragalactic Cepheids and the possible corrections a perfect point-spread function modeling, blending could still because of their particular properties (i.e., metallicity). A clear be present. The effect can be the same as in spectroscopic bi- example is the Andromeda galaxy (M 31), where the first iden- naries, where the individual components cannot usually be re- tification of Cepheids was already performed by Hubble (1929). solved from ground-based images. After the observations of Baade & Swope (1965, and references therein), few efforts have been dedicated to further analyze the When studying extragalactic Cepheids the spatial resolu- Cepheid population in M 31. tion decreases linearly with the distance of the host galaxy This trend has changed in recent years with the emer- and, as a consequence, the number of possible blends increases. gence of new observational capabilities. Several variability sur- Small blending values are expected for the Large Magellanic veys have started to study the stellar content in M 31 (Macri Cloud (LMC), where individual Cepheids can be resolved from 2004, and references therein) and other Local Group galaxies neighboring stars, even from ground-based observations. The situation changes in M 31 and M 33, where mean blending . Based on observations made with the Isaac Newton Telescope op- values of 0 18 mag and 0 16 mag, respectively, have been ob- erated on the island of La Palma by the Isaac Newton Group in the tained (Mochejska et al. 2004, hereafter MMSS04). These re- Spanish Observatorio del Roque de los Muchachos of the Instituto de sults would imply, when extrapolated to more distant galaxies, Astrofísica de Canarias. a downward revision of the Hubble constant between 5% to Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20077960 848 F. Vilardell et al.: A comprehensive study of Cepheid variables in the Andromeda galaxy 10% (as explained by Gibson et al. 2000). By contrast, Bresolin 14 et al. (2005) found an upper limit on blending in NGC 300 (at 15 ∼2Mpc)of0.04 mag. In addition, Gibson et al. (2000) showed ff that the systematic e ect of blending on farther galaxies (be- 16 tween 4 and 25 Mpc) is almost negligible. Therefore, results ob- tained so far seem to indicate that blending has an important 17 contribution when observing Local Group galaxies and dimin- ishes when observing distant galaxies. Gibson et al. (2000) ex- 18 plained such behavior as a consequence of the background levels in M 31 and M 33 (with a large number of stars detected around V 19 Cepheid variables) not being representative of the more distant galaxies (where the background levels are the result of several 20 unresolved sources). Because of the importance of the subject, a comprehensive study of the effect of blending on Cepheid dis- 21 tance determinations is highly desirable. One of the most recent variability surveys in M 31 was car- 22 ried out by us and was centered on the North-Eastern quadrant 23 of the galaxy (Vilardell et al. 2006, hereafter Paper I). Our main goal was to detect eclipsing binaries, although a sample 24 of 416 Cepheids also resulted from the reduction and analysis -0.5 0 0.5 1 1.5 2 2.5 3 process. The large number of detected stars and the high quality B-V of the resulting light curves have allowed the current study of Fig. 1. Color–magnitude diagram for 37 241 objects with photometric Cepheid properties in M 31. errors below 0.1 mag in the reference catalog of Paper I (grey dots) and the 416 identified Cepheids (black circles). Objects above the dashed line are saturated. Error bars on the left indicate the mean error, in mag- 2. Photometric sample nitude and color, at different magnitude values. The photometric survey in Paper I was carried out with the 2.5 m Isaac Newton Telescope (INT) at La Palma (Spain) over shallow surveys while, on the other hand, the distribution of the the course of five observing runs (between 1999 and 2003). observations can make the identification of long period Cepheids An implementation of the difference image analysis technique difficult. (Wozniak 2000) was used in the reduction process to automati- To evaluate the observational biases, the 416 Cepheids cally detect 3964 variable stars with ∼250 observations per band from Paper I were compared with the 420 Cepheids in M 31 (B and V). The analysis of variance algorithm (Schwarzenberg- of the GCVS (Samus et al. 2004). Both samples have al- Czerny 1996) was used to identify periodic variable stars. The most the same number of stars, but the two period distribu- folded light curves were visually inspected, which led to the tions (Fig. 2) are found to be largely different, as demonstrated identification of 437 eclipsing binaries and 416 Cepheids. by the Kolmogorov-Smirnov tests, which provide values of The photometric measurements were transformed to the stan- around 10−13. The origin of the observed difference can be ex- dard system through the observation of several Landolt (1992) plained by two main factors. Firstly, the Paper I sample is known star fields, obtaining magnitude errors between 0.03 and to have a bias for the longest period Cepheids because of the 0.11 mag. Finally, the phase-weighted mean magnitudes (Saha observational window function (see Paper I for further infor- & Hoessel 1990) of the Cepheid sample were determined for mation). And secondly, the GCVS presents an important bias each passband (B and V). Figure 1 shows the color-magnitude at short periods because the GCVS is shallower than our survey diagram of the observed stars with good photometric precision. and faint Cepheids are missing. Furthermore, both Cepheid sam- Cepheid variables are highlighted. ples in M 31 can be compared with the MW sample of the David Dunlap Observatory1 (DDO, Fernie et al. 1995). As can be seen (Fig. 2), the period distributions of Paper I and the DDO sample 3. Period distribution are very similar (with a Kolmogorov-Smirnovtest value of 0.49). Only a slight difference is observed at long periods, possibly be- The period distribution of Cepheids in Local Group galaxies has cause of the observational bias in the Paper I sample.